1. Field of the Invention
The present invention relates to an image pick-up device such as electronic still camera and digital camera including a solid state image sensing element, and more particularly to an image pick-up device which can produce an image signal having a wider dynamic range than that of a solid state image sensing element. The present invention also relates to a record medium having recorded thereon a computer readable program for controlling an imaging operation of the above mentioned image pick-up device.
2. Description of the Related Art
In the image pick-up device of the kind mentioned above, since the solid state image sensing element has a relatively narrow dynamic range compared with a conventional photographic film, when a subject situating on a relatively bright background is to be picked-up under such an exposure time that the subject is picked-up with a proper exposure amount, the background might be picked-up with an overexposure amount and an image signal corresponding to the background might be saturated. On the other hand, when the subject is picked-up under such an exposure time that the bright background is picked-up with a proper exposure amount, the subject might be picked-up with an underexposure amount and might become too dark. In order to increase a dynamic range of the solid state image sensing element, it has been proposed a known image pick-up device, in which an object is picked-up twice with different exposure amounts, and the thus obtained two image signals are synthesized by simply adding these image signals each other or by partially exchanging or combining these image signals in a mosaic fashion.
In the former solution in which the two image signals obtained with different exposure amounts are synthesized by simply adding them, when a subject moves or a camera moves to produce a relative positional shift between successively picked-up two image signals, S/N is decreased at a portion which is subjected to the movement and pseudo-color or pseudo-edge might be produced in a synthesized image signal.
In order to remove such a problem, in Japanese Patent Application Laid-open Publication Kokai Hei 2-280585, it has been proposed to derive a synthesized image signal in such a manner that a difference between the two image signals is derived and an absolute value of said difference is subtracted from the synthesized image signal obtained by merely adding the two image signals. However, in this case, since the image shift portion in the synthesized image signal is removed by the subtractive correction, when a subject moves, a configuration of the subject within a synthesized image might be deformed, and therefore the synthesized image has a feeling of strangeness as compared with an image formed on a conventional photographic film.
The above problem also occurs in the later solution, in which the two image signals are synthesized by combining or exchanging them in a mosaic fashion. That is to say, as shown in FIG. 13, when a too bright portion B (e.g. background) in a first image A picked-up with a large exposure amount is replaced by a corresponding portion D in a second image C picked-up with a small exposure amount after adjusting a gain of said portion D to derive a synthesized image E, if a subject F moves from right to left in FIG. 13 during a time interval between successive image picking-up operations, a part of the subject F in the second image C picked-up with small exposure amount corresponding to the movement is also synthesized as a part G in the synthesized image E. Then, the subject is partially seen double, and therefore a feeling of strangeness remains.